1. Field of the Invention
The present invention relates to a fixing unit for use in an image forming apparatus using the electrophotography.
2. Description of the Related Art
In the image forming apparatus such as a copying machine or a printer using the electrophotography, a photosensitive body (photosensitive drum) formed like a drum is uniformly charged, and exposed to light under control based on the image information to form an electrostatic latent image on the photosensitive drum. And this electrostatic latent image is made a visible image (toner image) by the toner, and the toner image is transferred onto the recording sheet, and fixed by a fixing unit to form an image.
The fixing unit for use in such image forming apparatus has a heating source disposed inside a cylindrical metallic core, and is composed of a fixing roll having a heat resistant elastic layer and a mold releasing layer on the metallic core and a pressure roll placed in contact under pressure with the fixing roll and having a heat resistant elastic-layer and a mold releasing layer of a heat resistive resin film or heat resistive rubber film on the metallic core. And the recording sheet bearing an unfixed toner image is passed between the fixing roll and the pressure roll, and the toner image is fixed on the recording sheet by heating and pressing the unfixed toner image. Such fixing unit is called a roll-nip type, and commonly widely employed.
When the fixing unit of the roll-nip type is sped up, it is required to widen the nip width in proportion to a fixing speed to supply a sufficient heat amount to the toner and the recording sheet. To widen the nip width, a method for increasing the load between the fixing roll and the pressure roll, a method for increasing the thickness of is an elastic body, or a method for increasing the roll diameter may be taken.
However, in the method for increasing the load or the method for increasing the thickness of the elastic body, since the shape of nip width becomes uneven along the roll shaft due to flexure of the roll, there is a problem with the image quality that irregular fixing or crumpled paper occurs. Also, in the method for increasing the roll diameter, there is a problem that the apparatus size is increased, and the time (warm-up time) required to heat the roll from room temperature to the fixable temperature is longer.
Thus, to solve these problems, and realize the fixing unit to cope with the high speed image forming apparatus, the present applicant offered a technology concerning the fixing unit which includes a fixing roll having an elastic member covered on the surface and an endless belt stretched around plural support rolls, the endless belt being wound in a predetermined angular region around the fixing roll to form a nip area between the endless belt and the fixing roll, wherein a greater pressure is applied locally to the exit of the nip area than any other portion of the nip area to distort the elastic member on the surface of the fixing roll (e.g., refer to Japanese Patent No. 3,084,692).
Moreover, the present applicant offered a technology concerning the fixing unit which includes a rotatable fixing roll having the elastically deformable surface, an endless belt that can run in contact with the fixing roll, and a pressure pad disposed in non-rotated state inside the endless belt, in which the endless belt is contacted under pressure with the fixing roll by the pressure pad to form a contact face with the fixing roll, and to provide a belt nip for passing the sheet between the endless belt and the fixing roll, and the surface of the fixing roll on the exit side of the sheet is elastically deformed locally (e.g., refer to Japanese Patent No. 3,298,354).
With the technology as described in the Japanese Patent No. 3,084,692, the endless belt stretched around the plural rolls is contacted to form a belt nip. Also, with the technology as described in the Japanese Patent No. 3,298,354, the endless belt is contacted under pressure with the fixing roll using the pressure pad to form a belt nip. By employing such constitution, the width of the belt nip formed by the fixing roll and the endless belt can be easily greater than the conventional width of the roll nip between the fixing roll and the pressure roll, whereby it is possible to speed up the apparatus, and reduce the size of the apparatus.
Particularly, since the heat capacity of the endless belt contacted under pressure with the fixing roll is small, the heat conducted from the fixing roll is difficult to radiate. Therefore, even if the rotation of the fixing roll is started, the heat amount taken away from the fixing roll to the endless belt is relatively small, so that the efficiency of heat used to fuse the toner is increased, with the advantage that the toner is fixed more excellently.
However, in the fixing units (generically referred to as a “belt nip type”) as described in the Japanese Patent Nos. 3,084,692 and 3,298,354, when the image forming apparatus is sped up to require the fixing process to be made for a number of recording sheets fed consecutively in a short time, a so-called “temperature droop” phenomenon occurs in which the surface temperature of the fixing roll temporarily drops at the rising time of the image forming apparatus. This temperature droop phenomenon is caused because the elastic layer made of silicone rubber covered around the metallic core of the fixing roll acts as a thermal resistor, producing a time lag for the heat to conduct to the surface of the fixing roll even if a sufficient heat amount is supplied from inside the fixing roll. Particularly, in a thick paper having great heat capacity, the heat amount taken away from the surface of the fixing roll is so great that the temperature droop tends to increase. Therefore, when the image forming apparatus is further sped up, there is a new problem that a fixing failure is likely to occur on some recording sheets until the surface temperature of the fixing roll is recovered.
Moreover, since the toner image is borne on the surface of the recording sheet, the toner image is fused by heat on the side of the fixing roll, so that the recording sheet and the surface of the fixing roll easily adhere to each other. Therefore, in the fixing unit of belt nip type, the fixing roll is formed with the mold releasing layer on the surface thereof to reduce adherence between the recording sheet and the surface of the fixing roll. An elastic member on the fixing roll surface on the exit side of the nip portion is locally elastically deformed to cause a down curl on the recording sheet, and peel the recording sheet from the fixing roll more easily. However, to speed up the image forming apparatus, it is required to peel the recording sheet passing through the nip portion at high speed from the fixing roll stably, because once a peeling failure occurs to cause a paper jam, a number of following recording sheets may be impaired due to the influence of the jam.